Continued lithographic pattern density scaling depends on aggressive overlay error reduction.1,2 Double patterning
processes planned for the 22nm node require overlay tolerances below 5 nm; at which point even sub-nanometer
contributions must be considered. In this paper we highlight the need to characterize and control the single-layer
matching among the three pattern placement mechanisms intrinsic to step&scan exposure - optical imaging, mask-to-
wafer scanning, and field-to-field stepping. Without stable and near-perfect pattern placement on each layer,
nanometer-scale layer-to-layer overlay tolerance is not likely to be achieved. Our approach to understanding onwafer
pattern placement is based on the well-known technique of stitched field overlay. We analyze dense
sampling around the field perimeter to partition the systematic contributors to pattern placement error on
representative dry and immersion exposure tools.